[Python-Dev] PEP 525

[Guido van Rossum]

On Wed, Aug 24, 2016 at 8:17 AM, Yury Selivanov <yselivanov.ml at gmail.com>
wrote:

> On 2016-08-23 10:38 PM, Rajiv Kumar wrote:
>
>> I was playing with your implementation to gain a better understanding of
>> the operation of asend() and friends. Since I was explicitly trying to
>> "manually" advance the generators, I wasn't using asyncio or other event
>> loop. This meant that the first thing I ran into with my toy code was the
>> RuntimeError ("cannot iterate async generator without finalizer set").
>>
>> As you have argued elsewhere, in practice the finalizer is likely to be
>> set by the event loop. Since the authors of event loops are likely to know
>> that they should set the finalizer, would it perhaps be acceptable to
>> merely issue a warning instead of an error if the finalizer is not set?
>> That way there isn't an extra hoop to jump through for simple examples.
>>
>> In my case, I just called
>>     sys.set_asyncgen_finalizer(lambda g: 1)
>> to get around the error and continue playing :) (I realize that's a bad
>> thing to do but it didn't matter for the toy cases)
>>
>
> Yeah, maybe warning would be sufficient.  I just find it's highly unlikely
> that a lot of people would use async generators without a loop/coroutine
> runner, as it's a very tedious process.
>

Heh, I had the same reaction as Rajiv. I think the tediousness is actually
a good argument that there's no reason to forbid this. I don't even think a
warning is needed. People who don't use a coroutine runner are probably
just playing around (maybe even in the REPL) and they shouldn't get advice
unasked.

Would it be possible to print a warning only when an async generator is
being finalized and doesn't run straight to the end without suspending or
yielding? For regular generators we have a similar exception (although I
don't recall whether we actually warn) -- if you call close() and it tries
to yield another value it is just GC'ed without giving the frame more
control. For an async generator there are two cases: either it tries to
yield another value (the first time this happens you can throw an error
back into it) or it tries to await -- in that case you can also throw an
error back into it, and if the error comes out unhandled you can print the
error (in both cases actually).

It's probably to specify all this behavior using some kind of default
finalizer (though you don't have to implement it that way).

Hopefully there will be other discussion as well, otherwise I'll have to
accept the PEP once this issue is cleared up. :-)

-- 
--Guido van Rossum (python.org/~guido)
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